The field of art to which this invention is directed is hydrogenation of carboxylic acid alkyl esters.
Fatty alcohols, i.e., aliphatic, predominantly linear, primary alcohols with chain lengths of more than 8 carbon atoms are important intermediates in the chemical industry. They are used preferably for production of tensides, such as fatty alcohol sulfates, polyglycol ethers, or polyglycol ether sulfates. Fatty acids or fatty acid esters, such as mixtures of different chain lengths found in natural fats and oils, are important raw materials for their production. They are converted to fatty alcohols by catalytic hydrogenation under pressure.
The hydrogenation reaction is carried out as a suspension hydrogenation, as a gas-phase hydrogenation, or in the trickle phase. Sufficiently high reaction rates are attained only at pressures above 250 bar and temperatures above 200.degree. C. As a rule, the triglycerides are transesterified by known methods before the hydrogenation with alcohols, especially with methanol, which esterifies the free fatty acids.
Hydrogenation of fatty acid methyl esters is characterized by a trend toward performing the reaction in the trickle phase.
The esters contain a more or less high residual concentration of free carboxylic acids, depending on whether or not they are subjected to purification by distillation before the hydrogenation. Choice of a suitable hydrogenation catalyst is decisively influenced by the purity of the esters used. At present, catalysts based on copper-chromium or copper-zinc have proved to be effective in industrial applications.
Aside from the hydrogenating activity, one important characteristic for evaluation of catalyst effectiveness is selectivity. When the known catalysts are used to hydrogenate fatty acid esters in the trickling phase, methane, higher hydrocarbons, and dialkyl ethers are formed. These by-products are formed at relatively high concentration, depending on the reaction temperature needed to attain a sufficiently high reaction rate. They must be removed from the reaction mixture by costly separation operations.
A process for production of a copper catalyst suitable for synthesis of methanol is known from German A-29 28 435, which is equivalent to GB 2,025,25. Along with copper oxide, it also contains zinc oxide, aluminum or chromium oxide, and manganese oxide. Use of chromium oxide is questionable, though, for environmental reasons. Aluminum oxide is not needed in catalysts for production of higher alcohols. Also, the methanol synthesis is usually carried out at pressures no greater than 20 bar.
A process for production of catalysts for synthesis of C.sub.2 -C.sub.5 alcohols is known from DD A-284 167. The catalysts used are based on oxides of copper, zinc, manganese, and/or aluminum. The preferred catalysts contain about 70% copper.
The object of the present invention was to avoid the disadvantages of the known catalysts and to establish a catalyst and a process for production of alcohols by catalytic hydrogenation of carboxylic acid alkyl esters, characterized by distinctly improved selectivity.